NFG Games + GameSX

The following information is kind contribution by our forum members Lydux, BlueBMW and RobIvy64. Thank you guys for another GameSX exclusive article !

For this repair the following tools will come in handy:

• Multimeter with buzzer
• Elenco LP-560 or the more advanced Elenco LP-900 logic probe

Both of these items are cheap and can be easily found on eBay.

• If the sound is coming out distorted or at a very low volume, the first thing you can try is replacing all capacitors.
• If that does not help, you could have a bad amplifier on Op-Amps (MC3403) or the dual channel DAC (YM3012). It could also be a power regulation problem on amplifier part. You can check this with a voltmeter on op-amp (pin 4 and 11). They should be +12v and -12v. If there is a PSU failure on -12v rail, it will cause distortions.
• Please note that the acid from leaked capacitors can eat up traces above and below the solder mask and traces which look fine can be corroded and broken below it. Please test all suspected traces for continuity.

MC3403 Op-Amp

It is a regular quad op-amp in SMD SO14 package.

• Original is a MC3403
• Compatible devices are TL074ACD which is preferred over LM324.

There are 3 Op-Amps:

• IC39 - Responsible for OPM DAC internal buffer amplification and first stage of stereo FM channels pre-amplification (Top)
• IC7 - Responsible for second stage of FM channels pre-amplification, ADPCM output amplification and channel mixing for internal speaker (we haven't checked so far) (Bottom)
• IC106 - NJM2073. It's a dual opamp, used for headphone amplification and in most cases can be safely ignored.

The circled test points in the right picture can become corroded and broken. Please test them for continuity.

If there are no broken traces and replacing the appropriate Op-Amp does not solve your problem then the following IC's could be faulty:

YM2151 FM synth	YM3012 DAC

• There is a RESET pin on YM3012 (pin 7 = ICL#) which should stay to a 1 logic (High) on normal operation. If stuck to a logic 0, it will keep initializing both channels, so no analog output.
• You can also check DAC amplification (the part at the right of YM3012), and channels preamplifications. Relevant components are all ceramic precision capacitors (C91, C28 and C29). These are rated at the lowest possible tolerance (B = ± 0.1 pF), this is very important.
• Also, transistors Q4 an Q5 may be faulty, or the diode D7 could be shorted.

Here is a flowchart which was kindly contributed by Lydux

Start with isolating the faulty block by testing each output, following the chain : Internal speaker =⇒ Headphone =⇒ Back connector (A0)

**No sound at all, on any output :**
  *Using a voltmeter, x68k powered on, check for power supply on opamps. If one present wrong voltage, check your PSU.
  *Change opamp IC7.

**Only Internal speaker and headphone present issues :**
  *Change opamp IC7 and volume potentiometer.

**Only headphone present issue :**
  *Change opamp IC106.

**Missing FM sound, on any output :**
  *Following the schematic, check for continuity between YM2151 and YM3012. Any broken trace will cause the DAC to not receive its data. So no output. Fix this by soldering wire between involved pins.
  *Using a voltmeter, x68k powered on, check for power supply on opamp. If one present wrong voltage, check your PSU.
  *Using the logic probe, x68k powered on, test for the following pins :
    *pin 7 of YM3012  and pin 3 of YM2151 should be high, if not that's IOSC/PEDEC issue.
    *pin 24 of YM2151 should pulse, if not that's IOSC/PEDEC issue.
    *pins 2, 4, 5 and 6 of YM3012 should pulse, if not change YM2151.

**All outputs present unexpected FM sound behavior (distorsions, low volume, saturation, ...) :**
  *Using a voltmeter, x68k powered on, check for power supply on opamp. If one present wrong voltage, check your PSU.
  *(Compact models only) Using an ohmmeter, x68k powered off, check for resistivity between pin 13 of IC39 and + pin of C31. Do as well between pin 7 of IC39 and + pin of C28. The indicated value should be < 100 ohm. If more or open circuit, the 2 test points near to IC7 pins starting to get corroded (see picture), and need to be cleaned. Soldering a simple wire between involded pins will also do the job.
  *Change opamp IC7 and IC39.
  *Remove transistors Q# and Q# and test using AO connector. If sound is ok, change Q#, Q# and D#. (#=Need Compact reference names)
  *If the issue always remains, it's time to change some capacitors. Start by changing precision capacitors. Then, full recap the whole audio part.
  *The DAC is faulty, change YM3012.

**Missing ADPCM sound on all output :**
  *Using a voltmeter, x68k powered on, check for power supply on opamp. If one present wrong voltage, check your PSU.
  *Change opamp IC7.
  *We need more investigations here...

For further troubleshooting please use the following schematics which have been kindly contributed by Lydux and updated by BlueBMW

Keep the scene alive !